Boiling water nuclear reactors (BWRs) typically include a reactor core located within a reactor pressure vessel (RPV). A known RPV includes a substantially cylindrical shell. The shell, for example, can be about twenty feet in diameter and about seven inches thick. A substantially cylindrical core shroud is positioned within, and spaced from, the shell walls to restrain horizontal movement of the reactor core fuel bundles and provides flow separation between the downcomer and the upward core flow.
A bottom head knuckle, or junction, assembly forms an interface, or junction, between the cylindrical shell and a substantially disk shaped bottom head. Specifically, the bottom head knuckle assembly includes a shroud support subassembly and an annular forging having a top, cylindrical shaped end and a bottom, conical shaped end. The top, cylindrical shaped end of the forging is configured to be welded to one end of the RPV shell and the bottom, conical shaped end of the forging is configured to be welded to the disk shaped bottom head.
The shroud support subassembly includes a shroud support cylinder having an upper surface configured to be welded to the core shroud. An annular pump deck extends from an outer surface of the shroud support cylinder. Shroud support legs, if used, extend from the lower surface of the shroud support cylinder. The shroud support legs are welded to weld build-up pads formed on an inner surface of the annular forging.
The shroud in a BWR is a cylindrical, steel structure which surrounds the core of the reactor and, as explained above, provides a barrier to separate the upward flow of water through the core from the downward flow in the annulus between the shroud and the RPV. The shroud also provides support for the core by restraining the top guide grid and core plate which are considered as part of the core support structure.
The shroud support provides structural support for the shroud as well as provides a baffle between the annulus and the bottom of the shroud. Pumps penetrate the shroud support to create the downward flow from the annulus into the shroud/core.
Stress corrosion cracking has occurred in shrouds and other internal components of the BWRs. Preventative measures have and are being implemented to reduce the risk of cracking due to stress corrosion cracking. Due to the working environment (radiation, underwater) and the numerous welds, substantial effort is required to replace a shroud. For example, remote cutting, welding, and ultrasonic inspections are required in removing and replacing a shroud.
Some known BWRs have shrouds which are bolted in place and are designed to be removed. In addition, some known pressurized water reactors have a core barrel, which is similar to a shroud, supported in the pressure vessel by a bolted connection.
It would be desirable to provide a shroud which can be removed and replaced more easily than known shrouds. It also would be desirable to provide such a removable shroud which performs all the functions of the known shrouds.